An offset reflector antenna with low sidelobes

The corrugated conical horn is considered to be an ideal feed for low sidelobe reflector antennas because of its unique characteristics. Analysis is carried out to show that low flare horns are preferred over high flare horns to give rise to low sidelobe performance for a given offset reflector antenna system.     

A parabolic reflector with asymmetric low sidelobes

A technique for generating low sidelobes on one side of the main lobe for a parabolic reflector antenna is presented. The reflector system that generates such an asymmetric sidelobe pattern consists of a central parabola with two sections of offset parabolas situated at the top and bottom of the central parabola. By adjusting the positions and dimensions of the side reflectors, considerable suppression of the first few sidelobes can be obtained. Further, by similar adjustments it is possible to achieve suppression of sidelobes on both sides of the main beam, though this suppression is relatively less. The technique, though explained in detail with reference to a cylindrical geometry, is extended for a paraboloid of revolution. The analysis program utilized for the optimization of the side reflectors is based on physical optics current integration.     

A half blinder for reducing certain sidelobes in large horn reflector antennas

The use of blinders to reduce certain high sidelobes in the azimuth (transverse) plane radiation for horizontal (transverse) polarization is discussed. A half blinder is presented which reduces objectionable high sidelobes 8 to -62 dB, and maintains low sidelobes elsewhere.     

Radiation properties of the optimized offset-fed radial rib umbrella reflector antenna

The properties of the offset-fed radial rib umbrella reflector are analyzed. A simple formula for positioning the feed to maximize the gain is given. The best aim point for the feed is numerically determined to be very close to the rib center. The exponent of acos^{n} alphafeed pattern, evaluated for maximum gain, and the loss due to the gores is presented. It is shown that the offset configuration exhibits the same behavior and position of the gore-related sidelobe as the front-fed umbrella reflector.     

Losses, sidelobes, and cross polarization caused by feed-supportstruts in reflector antennas: design curves

In a symmetrical reflector antenna the feed system and its support struts block the aperture and thereby deteriorate the radiation characteristics. Simple design curves are presented for the efficiency reduction, the sidelobe levels, and the cross polarization caused by strut blockage. The results are obtained from an analytical study that includes the induced field ratio (IFR) of the struts. The most significant IFR values for struts with circular cross section are calculated and plotted in a way which makes them easy to use as design curves. The use of the design curves is demonstrated by an example     

A small dipole-fed resonant reflector antenna with high efficiency, low cross polarization, and low sidelobes

The computer-aided optimization of a small five-wavelength diameter reflector antenna with a center-supported dipole-disk feed is described. The primary radiation is controlled by using a patented beamforming ring to give low cross polarization and low sidelobes due to spillover. The efficiency is maximized by controlling and taking advantage of the multiple reflections between the feed and the reflector. This has inspired the name "resonant reflector antenna." The gain from the feed reflector resonances is so large that it compensates almost completely for the about 1 dB loss due to center blockage of the aperture.     

Defocusing loss for a log periodic-fed reflector

Log periodic designs are a well established wide-band antenna technology, but one in which the phase center travels along the structure as the frequency varies. When a log periodic antenna is used as a feed for a reflector antenna, the phase center location cannot be maintained at the reflector focus over the frequency range and defocusing results. The purpose of this analysis is to quantify the defocusing loss for a log periodicfed reflector. The analysis presented compares favorably with measured results.     

An offset-fed reflector antenna with an axially symmetric main reflector

A design method for an offset-fed, dual reflector antenna (Cassegrain type or Gregorian type) system with an axisymmetric main reflector is presented. Geometrical optics (GO) and the geometrical theory of diffraction (GTD) are used to find the surface-current density on the main reflector. A modified Jacobi-Bessel series (JBS) method is used to find the far-field pattern for the physical optics (PO) integral. In the defocused mode of operation, a new technique is developed to find the reflection point on the subreflector corresponding to the defocused feed and a general field point on the main reflector. Two sample systems are designed.     

S-resonator with an end reflector

An S-resonator with an end reflector (SRER) is analyzed in terms of its resonance conditions and directional properties. An S-resonator has a main fiber ring to which is attached an S-shaped fiber by means of two couplers. Compared to the basic S resonator (BSR), the SRER has a highly reflecting reflector at a free end of one of the coupler ports. Because of the complex recirculating routes in the two resonators, their characteristics depend strongly on the coupling coefficients of the couplers; the phase resonance condition for the subcavity can even be shifted from an even to an odd integer of π, or vice versa. The new resonator can offer a higher clockwise over counterclockwise circulating intensity ratio. This can be very useful in the design of a ring fiber laser without an isolator     

On an algorithm for analysis of the radiation patterns of dual reflector and segmented reflector antennas

Large reflector antennas, from cost and weight considerations, appear to be the best configurations for achieving the very large apertures needed for antennas mounted on orbiting spacecraft. Radiation pattern calculations for such antennas are, in general, quite costly both in terms of computer time and the considerable memory required to perform the large surface integrations. The method presented here treats the large aperture as a set of small subapertures for which the radiation fields are computed separately, stored and then added with due regard to phase to yield the radiation pattern of the large reflector. Numerical methods developed to provide the illumination of each subaperture to avoid overlap of the aperture fields of adjacent subapertures and to simplify the surface integrations are discussed. The algorithm is straightforward and has considerable intuitive appeal. The methods of geometrical optics (GO) are used to calculate the aperture plane tangential field components; and electric vector potential is then used to compute the antenna radiation fields. The algorithm includes a set of options for different reflector surfaces. If high accuracy of the far sidelobe levels is required, a subroutine accounting for edge diffraction should be added to the algorithm given here. Calculations made with this algorithm are compared with calculations made by other methods and with measured patterns.     

Loss-budget versus comprehensive analysis of gain loss for microwave reflector antennas

Microwave reflector antennas are often analyzed by utilizing a loss-budget technique in which the gain degradations due to various perturbations are individually analyzed and then added together to yield a total antenna gain loss. The comprehensive analysis technique described shows that the loss-budget technique may yield erroneous results since it fails to take into account the possible interrelations of the perturbations.     

Pulse compression with very low sidelobes in an airborne rainmapping radar

Pulse compression allows a substantial reduction in the peak transmitted power of a radar and is attractive for spaceborne remote sensing applications. In the case of a downward looking rain measuring radar, however, the range sidelobes associated with surface return can mask return from rain and must be kept to a minimum. The authors describe the pulse compression system for the NASA/JPL Airborne Rain Mapping Radar. This system uses time-domain weighting of the transmitted pulse and is able to achieve a range sidelobe level of -55 dB or better in flight tests. This is significantly lower than other values reported in the open literature     

Radiation pattern of an offset hyperbolic reflector

The radiation pattern of a focus-fed offset hyperbolic reflector is determined by using the uniform geometrical theory of diffraction (UTD) and the uniform asymptotic theory of diffraction (UAT). The patterns predicted by these two theories are observed to differ considerably around incident and reflection boundaries. The effects of the slope diffraction as well as those of additional terms in the improved UTD solution are ignored     

Millimeter-wave folded reflector antennas with high gain, low loss, and low profile

Periodic and quasi-periodic structures, printed on a dielectric substrate, can be employed to control the reflection and transmission properties of incident waves as a function of structure geometry. Local variations of the element geometry on a substrate with backside metallization - resulting in respective variations of the reflection phase angle - can be used to design printed reflectarray antennas. The dual-polarization properties of such antennas, together with polarizing grids or slot arrays, can be exploited for the realization of compact, low-profile folded reflector antennas. Examples of some antennas of this type are presented, covering the 60 GHz range for communication and ISM applications, and 76 to 77 GHz for automotive radars.     

Suppression of range sidelobes in bistatic radars

Conventional weighting schemes give little or no suppression of some of the range sidelobes of a linear-FM rectangular pulse passed through a matched filter.     

Single reflector synthesis using an analytical gradient procedure

A very efficient iterative procedure for the synthesis of a single reflector antenna from far-field data under the assumptions of physical optics (PO) is discussed. A key feature of the approach is that an analytical gradient expression is used to modify the reflector shape in each iteration and a geometrical optics (GO) method is used to provide an initial solution     

Multiple-beam dual-offset reflector antenna with an array feed

A computer simulation is described for the production of multiple beams by a dual-offset reflector antenna with a feed consisting of a circular waveguide array. It is shown that acceptable efficiencies and low crosspolar levels can be achieved for two sample beams of a multibeam configuration by deriving the array excitation from the focal-region fields of the double-offset antenna.     

Effect of an arcjet plume on satellite reflector performance

The effect of an arcjet plume on the performance of satellite reflector antennas is studied. The arcjet plume is modeled as a weakly ionized plasma. The spatial permittivity distribution of the plume is approximated using the measured electron density profile and a cold plasma model. Geometrical optics is applied to determine the ray paths as well as the transmitted fields through the inhomogeneous plume. The ray optics results are compared against several exact solutions for scattering from inhomogeneous dielectrics, and good agreement is observed for sufficiently large scatterer size. The far-field antenna patterns of the reflector in the presence of the plume are calculated from the transmitted ray fields using a ray-tube integration scheme. For arcjet prototypes in the 1-kW class, the plume effect on the antenna performance is small. As the electron density increases, the main beam and sidelobe level gradually degrade. The main beam also tends to squint away from the plume region     

Preliminary announcement of an 85 percent efficient reflector antenna

This communication is our preliminary announcement and claim for achievement of 85 percent aperture efficiency using a dual reflector antenna. We are aware of no other reflector antenna having such aperture efficiency, and we invite the antenna community to comment, critique, or inform us of equal or better claims. A full report will be forthcoming; this brief note will supply only pertinent highlights.